The present invention relates to a system and a method for compressing digitized color image signals utilizing an error tolerant compression algorithm.
Image compression is used to reduce the amount of data needed to replicate the image with little or no reduction in quality at a later time. The requirement for reducing the data is driven by the need for quickly transmitting the image and by the demand to reduce the amount of storage space needed to store each image.
During the process of recording or transmitting a color image, compressed digital signal errors may be generated. A commercially viable system must be able to tolerate these errors so as to perform its compression and decompression functions without degrading the quality of the reconstructed image.
Prior art systems generally operate by separating a color image into its red, green and blue components or by converting these components into three color components denoted Y, I and Q. Each of these color components are then individually processed. One technique of processing these color components that is particularly relevant to the present invention is called "block truncation coding." An image is block coded by dividing an image frame into blocks and performing some operation, upon the picture elements in each block. Block coding was first applied to monochrome images, see, for example, representative U.S. Pat. Nos. 3,403,226 and 4,205,341 and a publication by E. J. Delp and O. R. Mitchell entitled "Image Compression Using Block Truncation Coding," IEEE Transactions on Communications, Vol. COM-27, No. 9, pp. 1335-1342, September 1979. Block coding was then applied to color images, see, for example, U.S. Pat. No. 4,319,267 entitled "Picture Coding and/or Decoding Equipment," by E. Mitsuya et al.
In the U.S. Pat. No. 4,319,267, the color image is separated into three color planes of image color components Y, I and Q representing the encoding of blocks of pixels.
The encoding provides each block with a gray level code that represents one typical gray level component of the pixels in the block and a resolution code. The patent also teaches the use of different block sizes for encoding the color components. The example given in the patent divides the Y color components representing an image frame into small blocks with the I and the Q color components being divided into larger blocks. The stated advantage of different block sizes for different color planes is enhancement of the coding efficiency without substantial reduction in the quality of a reconstructed image. In the example of the patent, the resolution code, obtained for the Y color plane, is equally employed in the coding of the I and the Q color planes. The stated advantage for this approach is that the amount of coded data needed for reconstruction is reduced as compared with the amount of coded data needed for the case where the resolution code of each color plane has to be transmitted or recorded.
If the speed of image transmission or recording is of prime importance, then the amount of signal compression has to be maximized with a resultant degradation in the quality of the reconstructed image. Also, with high speed transmission or recording, errors may be generated which can accumulate through each image frame causing further degradation of the reconstructed image.
The present invention is directed to an optimum compromise between the requirement for high speed color image compression and high tolerance to channel errors.